1. Field of the Invention
The present invention relates to a polyphenylene ether composition, and more particularly to a polyphenylene ether composition improved in processability.
2. Description of the Prior Art
Polypheneylene ether is a noteworthy engineering plastics material which is very useful on account of its outstanding electrical and mechanical properties, high heat distortion temperature, and self-extinguishing properties. On the other hand, it has some disadvantages. It is slightly brittle due to low impact resistance. It has a high melting point and a high melt viscosity, and therefore it requires a high molding temperature and molding pressure. This leads to the difficulty in its melt processing.
In order to improve the processability of polyphenylene ether, there was proposed resin blending. For example, Japanese Patent Publication No. 17812/1967 discloses the blending of polyphenylene ether with high-impact polystyrene resin. The composition obtained by blending is improved in processability and impact resistance, but the improvement in processability is not satisfactory.
There was proposed a way of improving the processability of polyphenylene ether by adding a plasticizer to polyphenylene ether. For example, according to Japanese Patent Publication No. 5220/1974, the processability of polyphenylene ether is improved by incorporating polyphenylene ether or a composition of polyphenylene ether and styrene resin with a compound selected from aromatic organic acid ester, polyester having aromatic groups, organic phosphoric ester having aromatic groups, and chlorinated aromatic hydrocarbon which are highly miscible with polyphenylene ether resin.
When incorporated with a plasticizer such as organic phosphate ester having aromatic groups, polyphenylene ether or a composition of polyphenylene ether and styrene resin is improved in processability but becomes extremely poor in thermal properties. This is attributable to the fact that the incorporated plasticizer disperses very uniformly into the matrix of polyphenylene or a composition of polyphenylene ether and styrene resin, lowering the glass transition point (Tg) of the matrix and consequently decreasing the heat resistance of the matrix.
The present inventors conceived that the additive should have the following characteristics if the processability (flowability) is to be improved without sacrificing the thermal performance (heat resistance).
(1) It should be highly miscible with the matrix at the time of processing or when the system is in the flowing state, so that it produces the effect of improving flowability like a plasticizer.
(2) It should separate from the matrix when the matrix is not flowing (at a temperature lower than the glass transition point of the matrix), so that it does not lower the Tg of the matrix. And yet it should have a certain degree of miscibility with the matrix, so that it keeps a certain level of interface bond strength and keeps the mechanical strength even when it has separated from the matrix.
In order to find the compound that satisfies the above two conditions simultaneously, the present inventors carried out extensive studies and concluded that such a compound should satisfy the following three conditions.
(a) It is a low-molecular compound having a constituent unit which is miscible with the matrix (polyphenylene ether and styrene resin) and works as a plasticizer at the time of processing or when the system is in the flowing state.
(b) It is a compound which crystallizes and separates from the matrix phase at a temperature lower than the processing temperature (or the Tg of the matrix).
(c) It is a compound which has a comparatively high polarity, so that it brings about with certainty the crystallization at a temperature lower than the processing temperature. It is a compound having a unit which has affinity for the matrix (polyphenylene ether and styrene resin) so that it keeps the interface bond strength with the matrix even when it crystallizes and separates from the matrix phase.